In magnetoresistive random access memory (MRAM) devices, the memory cells are typically programmed by magnetic fields induced by current carrying conductor lines such as copper lines or aluminum lines. Typically, two orthogonal interconnects are employed, with one positioned above the magnetic memory device and the second positioned below the magnetic memory device.
FIG. 1A depicts a portion of such a conventional MRAM 1. The conventional MRAM includes conventional orthogonal conductive lines 10 and 12, conventional magnetic storage cell having a conventional magnetic tunneling junction (MTJ) stack 30 and a conventional transistor 13. In some designs, the conventional transistor 13 is replaced by a diode, or completely omitted, with the conventional MTJ cell 30 in direct contact with the conventional word line 10. Use of a conventional MTJ stack 30 makes it possible to design an MRAM cell with high integration density, high speed, low read power, and soft error rate (SER) immunity. However, the conventional MRAM 1 might be fabricated with a memory cells using a variety of magnetic elements, such as an Anisotropic Magnetoresistance (AMR) element, a Giant Magnetoresistance (GMR) element, and a Magnetic Tunneling Junction (MTJ) stack.
The conductive lines 10 and 12 are used for writing data to the magnetic storage device 30. The MTJ stack 30 is located at the intersection of and between conventional conductive lines 10 and 12. Conventional conductive lines 10 and 12 are referred to as the conventional word line 10 and the conventional bit line 12, respectively. The names, however, are interchangeable. Other names, such as row line, column line, digit line, and data line, may also be used. The magnetic field for changing the orientation of the changeable magnetic vector is usually supplied by two conductive lines that are substantially orthogonal to each other. When electrical current passes through the two conductive lines at the same time, two magnetic fields associated with the current in the two conductive lines act on the changeable magnetic vector to orient its direction.
The conventional MTJ 30 stack primarily includes the free layer 38 with a changeable magnetic vector (not explicitly shown), the pinned layer 34 with a fixed magnetic vector (not explicitly shown), and an insulator 36 in between the two magnetic layers 34 and 38. The insulator 36 typically has a thickness that is low enough to allow tunneling of charge carriers between the magnetic layers 34 and 38. Thus, the insulator 36 typically acts as a tunneling barrier between the magnetic layers 34 and 38. Layer 32 is usually a composite of seed layers and an antiferromagnetic (AFM) layer that is strongly coupled to the pinned magnetic layer. The AFM layer included in the layers 32 is usually Mn alloy, such as IrMn, NiMn, PdMn, PtMn, CrPtMn, and so on. The AFM layer is typically strongly exchanged coupled to the pinned layer 34 to ensure that the magnetic vector of the pinned layer 34 is strongly pinned in a particular direction. The conventional MTJ stack 30 may also include a capping layer (not shown).
When the magnetic vector of the free layer 38 is aligned with that of the pinned layer 34, the MTJ stack 30 is in a low resistance state. When the magnetic vector of the free layer 38 is antiparallel to that of the pinned layer 34, the MTJ stack 30 is in a high resistance state. Thus, the resistance of the MTJ stack 30 measured across the insulating layer 34 is lower when the magnetic vectors of the layers 34 and 38 are parallel than when the magnetic vectors of the layers 34 and 38 are in opposite directions.
Data is stored in the conventional MTJ stack 30 by applying a magnetic field to the conventional MTJ stack 30. The applied magnetic field has a direction chosen to move the changeable magnetic vector of the free layer 30 to a selected orientation. During writing, the electrical current I1 flowing in the conventional bit line 12 and I2 flowing in the conventional word line 10 yield two magnetic fields on the free layer 38. In response to the magnetic fields generated by the currents I1 and I2, the magnetic vector in free layer 38 is oriented in a particular, stable direction. This direction depends on the direction and amplitude of I1 and I2 and the properties and shape of the free layer 38. Generally, writing a zero (0) requires the direction of either I1 or I2 to be different than when writing a one (1). Typically, the aligned orientation can be designated a logic 1 or 0, while the misaligned orientation is the opposite, i.e., a logic 0 or 1, respectively.
Although the MRAM 1 functions, one of ordinary skill in the art will readily recognize that it is desirable to reduce the current used in writing to the MTJ stack 30. The current typically used is on the order of several milli-Amperes for each of the conductive lines 10 and 12. Therefore, one of ordinary skill in the art will also recognize that a smaller writing current is desired for many memory applications.
FIG. 1B depicts a portion of a conventional magnetic memory 1′ that has a somewhat lower writing current. Similar systems are described in U.S. Pat. No. 5,659,499, U.S. Pat. No. 5,940,319, U.S. Pat. No. 6,211,090, U.S. Pat. No. 6,153,443, and U.S. Patent Application Publication No. 2002/0127743. The conventional systems and conventional methods for fabricating the conventional systems disclosed in these references encapsulate bit lines and word lines with soft magnetic cladding layer on the three surfaces not facing MTJ cell 11′ in a manner similar to that which is described below. In addition, many of the portions of the conventional memory depicted in FIG. 1B are analogous to those depicted in FIG. 1A and are thus labeled similarly. The system 1′ depicted in FIG. 1B includes the conventional MTJ cell 30′, conventional word line 10′ and bit line 12′. The conventional word line 10′ is composed of two parts: a copper core 11 and a soft magnetic cladding layer 9. Similarly, the conventional bit line 12′ is composed of two parts: a copper core 15 and a soft magnetic cladding layer 13.
The soft magnetic cladding layers 9 and 13 can concentrate the magnetic flux associated with I1 and I2 onto the MTJ cell 30′ and reduce the magnetic field on the surfaces which are not facing the MTJ cell 30′. Thus, the soft magnetic cladding layers 9 and 13 concentrate the flux on the MTJ that makes up the MTJ cell 30′, making the free layer 38 easier to program. The conventional MRAM 1′ can thus achieve a significant improvement in the write efficiency over the conventional MRAM 1.
Although these approaches work to a certain extent, one of ordinary skill in the art will readily recognize that a further reduction in current is desirable. In addition, in certain applications, particularly portable devices such as mobile phones, personal digital assistants, or palm top computers, battery life is an important design factor. A reduction in current could dramatically decrease the power consumption of the memory and, therefore, extend the life of the battery for the device.
Accordingly, what is needed is a method and system for providing a magnetic memory capable of being written using a lower current.